The present invention relates to a liquid crystal device used for light valves used in flat panel displays, projection displays, printers, etc.
The twisted nematic (TN) mode, disclosed by M. Schadt and W. Helfrich (e.g., Appl. Phys. Lett., vol. 18, no. 4, pp. 127-128 (Feb. 15, 1971), has been used as a representative mode for a nematic liquid crystal device extensively used for display devices using active elements such as thin film transistors (TFTs).
On the other hand, in recent years, liquid crystal displays according to an in-plane switching mode utilizing a lateral electric field and a vertical alignment mode have been proposed to improve the viewing angle characteristic, which has been problematic in conventional liquid crystal displays.
As described above, several liquid crystal drive modes are known for TFT display devices using nematic liquid crystals, but any drive mode has a slow response time of several tens of milliseconds or more, and an improvement in response speed is desired.
Some liquid crystal drive modes using chiral smectic liquid crystals have been proposed in recent years for improving the response speed of conventional nematic liquid crystal devices, including a short pitch-type ferroelectric liquid crystal mode, a polymer stabilization-type ferroelectric liquid crystal mode and a thresholdless anti-ferroelectric liquid crystal mode, all of which have been reported as realizing high speed responsiveness on the order of a submillisecond or less; these have not been commercialized.
On the other hand, we also have proposed a liquid crystal device wherein a liquid crystal material shows a phase transition series on temperature decrease of isotropic liquid phase (Iso.)xe2x80x94cholesteric phase (Ch)xe2x80x94chiral smectic C phase (SmC*) or Isoxe2x80x94SmC* causing a direct phase transition from Iso to SmC* substantially without intervening Ch phase so as to provide a monostable state at a position inside a chiral smectic one. At the time of phase transition of Chxe2x80x94SmC* or Isoxe2x80x94SmC*, a DC voltage is applied across a pair of electrodes sandwiching the liquid crystal to make uniform the smectic layer directions. As a result, it is possible to realize a liquid crystal device which allows a high speed response and gradation control, which also exhibits excellent motion picture quality and high luminance and which also allows mass production.
Such ferroelectric liquid crystals and antiferroelectric liquid crystals causing an inversion switch owing to spontaneous polarization are all liquid crystals assuming a chiral smectic phase. Thus, it is expected to realize a display device using a chiral smectic liquid crystal capable of solving the problem of slow response speed accompanying the conventional nematic liquid crystal.
As described above, it has been expected to utilize a smectic liquid crystal device exhibiting high speed responsiveness and gradational display characteristic as a next generation display device. At present, however, optimum device design, particularly optimum design of holding capacitance, has not been achieved.
Regarding the holding capacitance design for a liquid crystal having a spontaneous polarization, Takato has reported that it is important to provide as large a holding capacitance as possible for appropriately designing such a liquid crystal having a spontaneous polarization (AMLCD, vol. 97, pp. 29-). More specifically, assuming that liquid crystal molecules are not inverted within a gate-on period during active matrix drive using TFTs (TFT drive), a charge supplied to a device during the gate-on period is compensated by inversion of spontaneous polarization to result in a decease in charge remaining at the device, thereby causing a voltage drop. The voltage drop Vd caused by the inversion of spontaneous polarization can be approximately expressed by the following equation (1):
Vd=2xc3x97Ps""xc3x97S/(C1c+Cs)xe2x80x83xe2x80x83(1),
wherein Ps"" denotes a magnitude of actually inverted spontaneous polarization depending on a desired gradation level. For example, in the case of displaying a 50% gradational level, Ps"" is equal to half the spontaneous polarization (Ps) of the liquid crystal material. S denotes an area of the switching region; C1c denotes a capacitance of the liquid crystal layer, and Cs denotes a magnitude of the holding capacitance. From the above equation (1), a smaller Vd requires a larger holding capacitance (Cs).
On the other hand, we have noticed that in the above-mentioned device utilizing a monostability of a chiral smectic liquid crystal, when using a very large holding capacitance relative to the liquid crystal capacitance for a TFT drive, a small difference in liquid crystal layer thickness (cell thickness) results in a remarkable change in the voltage-transmittance characteristic.
In view of the above-mentioned problems, a principal object of the present invention is to provide a liquid crystal device using a liquid crystal having a spontaneous polarization, particularly one having the above-mentioned phase transition series, placed in a monostable alignment state, which device shows little fluctuation in V-T characteristic even with some fluctuation in cell thickness in an active matrix device, thus allowing a high yield of production.
According to the present invention, there is provided a liquid crystal device suitable for analog gradational display according to active matrix drive comprising: a pair of substrates disposed to have mutually opposing surfaces of which at least one has been subjected to a uniaxial aligning treatment, and a chiral smectic liquid crystal disposed between the substrates so as to form a matrix of pixels, each provided with an active element and electrodes for applying a voltage to the liquid crystal via the active element, wherein the liquid crystal is placed in such an alignment state that the average molecular axis of the liquid crystal is monostabilized at a first position under no voltage application, the average molecular axis of the liquid crystal is tilted under application of a first polarity of voltage at an angle a1 in a first direction from the monostabilized first position depending on a magnitude of the applied voltage, and the average molecular axis of the liquid crystal is tilted under application of a second polarity of voltage opposite to the first polarity at an angle a2 in a second direction opposite to the first direction from the monostabilized first position depending on a magnitude of the second polarity voltage, so that at least one of the tilt angle xcex11 and xcex12 has a tendency to increase at a larger liquid crystal layer thickness between the substrates.
In a preferred embodiment of the device, a holding capacitance Cs is coupled to each active element, satisfying a relationship of Csxe2x89xa61.5xc3x97C1c with a capacitance C1c of the liquid crystal, or such a holding capacitance is not coupled to each active element.